In general, the invention relates to integrated optics formed on a chip. In particular, the invention relates to a wavelength dispersive optical structure such as an arrayed waveguide demultiplexing/switching element.
Arrayed waveguide (AWG) multiplexers, also referred to as phasars, have become an important component in many optical systems, particularly in wavelength division multiplexing (WDM) telecommunication systems. A WDM network uses a single optical fiber to carry a multiplicity of optical carriers at different wavelengths, each modulated with its own data signal. Electronics and opto-electronics are generally limited to data rates of 10 to 40 gigabits per second (Gbs). In WDM, with the proper optical multiplexing and demultiplexing at the ends of the fiber, the electronics can be operated in parallel on the wavelength separated carriers to achieve better utilization of the fiber bandwidth, which may be as high as 300 terahertz (THz). WDM effectively multiplies the transmission capacity of the fiber by the number of optical carriers.
An example of a WDM telecommunication system is illustrated in FIG. 1. Multiple (N) electronic data channels enter a transmitter 10 and modulate separate optical emitters such as lasers 12 having N respective free-space output carrier wavelengths xcex1, xcex2, . . . xcexN. The number N of WDM channels is increasing to 64 and beyond. The wavelengths correspond to respective optical frequencies fi=c/xcexi, where c is the speed of light, resulting in N frequencies f1, f2, . . . fN. Conveniently, these frequencies are arranged in a WDM frequency comb having the neighboring wavelengths f1, f2, . . . fN separated by a substantially constant inter-channel spacing given by
xcex94fS=fi+1xe2x88x92fi.xe2x80x83xe2x80x83(1) 
A typical frequency spacing xcex94fs is 100 GHz. Since the frequencies are narrowly spaced, it is sometimes easier to visualize the spacing in terms of wavelength spacing given approximately by                               Δ          ⁢                      xe2x80x83                    ⁢                      λ            S                          ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                            λ              2                        c                    ⁢                      xe2x80x83                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢                                    f              S                        .                                              (        2        )            
In the case of a central wavelength of 1550 nm and a channel frequency spacing of 100 GHz, the channel wavelength spacing xcex94xcexS is about 0.8 nm. An optical wavelength-division multiplexer 14 combines the optical signals of different wavelengths and outputs the combined signal on a single optical fiber 16. An optical receiver 20 includes a wavelength-division demultiplexer 22 which divides its received signals according to their optical wavelength to N optical detectors 24 according to the same wavelength allocation xcex1, xcex2, . . . xcexN. In view of the reciprocity usually exhibited in passive systems, a wavelength-division demultiplexer may be substantially identical to a wavelength-division multiplexer with a reversal of their inputs and outputs.
Additionally, an optical addidrop multiplexer (ADM) 30 may be interposed on the optical path 16 between the transmitter and the receiver 20. The optical add/drop circuit 30 removes from the optical signal on the fiber 16 one or more wavelength channels at wavelength xcexAD and inserts back onto the fiber 16 an optical data signal perhaps containing different information but at the same optical carrier wavelength xcexAD. The ADM 30 is typically implemented with technology closely resembling the WDMs 14, 22.
All-optical networks have been proposed in which a distributed network has many nodes each including a transmitter 10 and receiver 20 and which are linked by a functionally passive network which routes the signals between the nodes according to their wavelengths. The routing elements in such an all-optical network require switching elements similar to the ADM 30.
In order to maximize the transmission capacity of the optical fiber 16 and to utilize the usable bandwidth of certain elements such as erbium-doped fiber amplifiers, the wavelength channels xcex1, xcex2, . . . xcexN should be placed as closely together as possible with a minimum channel spacing xcex94xcexS. In advanced systems, this inter-channel spacing xcex94xcexS is 1 nm or less for signals centered around 1300 or 1550 nm, the preferred bands for silica fiber. Such closely spaced WDM networks are referred to as dense WDM networks (DWDM).
The network design described above may be subject to a problem arising from the fact that the operation of the transmitter 10, receiver 20 and intermediate node 30 are all referenced to the same set of WDM wavelengths xcex1, xcex2, . . . xcexN. However each of the distributed elements must provide its own wavelength calibration. Due to environmental and aging effects, the wavelength calibration settings at one element is likely to differ from those at other elements. In view of the close spacing of the optical channels, any miscalibration between network elements is likely to produce inter-channel interference.
For an optimized optical system, the fiber 16, the WDMs 14, 22, and the ADM 30 are typically designed to be single-mode at least at their ports for the optical wavelengths being used. Although each of the lasers 12 is likely emitting light across an exceedingly narrow bandwidth, the single-mode response of the frequency sensitive elements 14, 22, 30 usually has a wavelength (frequency) characteristic that approximates a gaussian distribution about the center wavelength xcex0 of the channel F(xcex)=exp(xe2x88x92(xcexxe2x88x92xcex0)/∂xcexG). The value of the gaussian passband ∂xcexG can be fairly freely chosen for present day fabrication techniques. However, the value of the passband is subject to countervailing restraints. For dense WDM systems, the inter-channel spacing xcex94xcexS is made as small as possible. The gaussian passband ∂xcexG must be substantially smaller than the inter-channel spacing xcex94xcexS to avoid interference between channels. On the other hand, the frequency characteristics of the lasers 12 and other frequency-sensitive elements are subject to permanent or temporary variations. If the passband ∂xcexG is made too small, the peak is very narrow and small variations in wavelength away from the peak wavelength xcex0 cause operation to shift to the sides of the peak, thereby degrading the signal strength. That is, for a strong signal the passband ∂xcexG should be made as large as possible to provide a broad top of the peak.
Amersfoort et al. have already recognized these problems, as disclosed in U.S. Pat. No. 5,629,992, incorporated herein by reference in its entirety. This patent describes arrayed waveguide gratings, also called phasars, of the sort described by Hunsperger et al. in U.S. Pat. No. 4,773,063, and by Dragone in U.S. Pat. Nos. 5,412,744 and 5,488,680. In particular, Amersfoort et al. describe a WDM phasar 40 exemplified in the schematic illustration of FIG. 2. A single-mode waveguide 42 is coupled to one end of a multi-mode waveguide 44 of length chosen to produce a doubled image of the radiation from the single-mode waveguide 42 at a port 46 on one side wall 48 of a first free-space region 50. The width of the single-mode waveguide 42 is approximately equal to the wavelengths of the light it is carrying, taking into account the refractive index, to within near-unity constants. The multi-mode waveguide 44 has a larger width. The multi-mode waveguide 44 acts as a multi-mode interferometer (MMI). Multiple single-mode array waveguides 52 are coupled to ports on the other side of the first free-space region 50 in the form of a star coupler. The array waveguides 52 are coupled on the other end to one side of a second free-space region 54. The array waveguides 52 have lengths with predetermined length differences between them to act as an arrayed waveguide grating (AWG), operating similarly to a planar diffraction grating. Single-mode output waveguides 56 are coupled to the other side of the second free-space region 54 along its output wall 58. The AWG causes the multi-wavelength signal from the input waveguide 42 to be wavelength demultiplexed on the respective output waveguides 56. Because of the reciprocal nature of the device, the roles of input and output can be reversed so that the same structure can be used as a wavelength multiplexer or as a wavelength demultiplexer or as a demultiplexer. An AWG is one example of a wavelength-dispersive optical device. The placement and number of waveguides contemplated by Amersfoort et al. are wider than the example of a single input presented below.
The illustration of the optical circuit of FIG. 2 is somewhat schematic. The illustrated structure provides horizontal optical waveguiding and generally includes a high-index waveguide surrounded on four sides by a low-index cladding. The vertical waveguiding is typically accomplished by a layered or slab structure of low-index layers sandwiching a high-index waveguide layer. However, the two waveguiding structures can be combined in, for example, a ridge waveguide.
The MMI 44 is designed to convert the narrow gaussian optical field carried on the single-mode input fiber 42 into a significantly broader non-gaussian optical field at the interface 46 to the first free-space region 50. The wavelength characteristic of the free-space between the multi-mode waveguide 44 and the rest of the phasar 40 is therefore also flattened. As a result, with the use of the multi-mode interference filter 44, it is possible to obtain a phasar with a narrow wavelength passband ∂xcex, as illustrated in the spectrum 60 of FIG. 3, but with a flattened top 62. The resultant transmission function T(xcex) is subject to smaller variations in response to small wavelength variations about the central values. However, the MMI solution of Amersfoort et al. is physically limited by a fixed relationship between the flatness and the passband ∂xcex since the illustrated spectra represent the sum of the two lowest lateral modes.
Somewhat similar results can be obtained using a Y-coupler which divides the single-mode input on input waveguide 42 between two single-mode waveguides entering the first free-space region 50. A widened signal is presented to the output waveguides 56, thus flattening the response.
The performance of any passband flattening technique similar to MMI or Y-branch is constrained by the fact that the desired rectangular response can only be approximated due to the finite slope or roll-off that can be obtained. The various techniques are differentiated by how well they approximate the optimum response. The optimum response corresponds to an optimum distribution in the output field. For a Y-branch design, the output field is constructed by the interference of two guide modes. For an MMI design, usually more than two guides modes are used, but the number is limited by enlarging sizes and increasing sensitivity to variations in wavelength and fabrication parameters.
In U.S. patent application Ser. No. 09/430,836, filed Nov. 1, 1999, now U.S. Pat. No. 6,289,147, incorporated herein by reference in its entirety, Bulthius et al. have disclosed an arrayed waveguide grating in which a Mach-Zehnder interferometer divides the input signal and introduces an optical length difference between them equal to the free spectral range of the phasar 40 before inputting them side by side to the multi-mode interference filter 44. This design compensates the movement of the Gaussian image in the output focal plane of the array waveguide grating and does not require transformation of the field, which for Y-branch and MMI results in a 2 dB power penalty. This design, however, requires that the Mach-Zehnder interferometer provides precisely 50:50 power splitting and a precise phase difference between the two signals. Such precision renders the fabrication to be difficult.
Another design for flattening the passband of a phasar, as disclosed in Japanese Laid-Open Patent Publication 9-298228, uses a linearly or parabolically shaped waveguide section between the single-mode waveguide and the free-space region. While the parabolic embodiment seems to achieve remarkable results, fabricating such a structure is considered very difficult.
Accordingly, it is desired to provide a phasar having improved flatness in its spectral response without sacrificing a narrow passband. It is further desired that this improvement be achieved with a design not requiring precise fabricational tolerances.
The invention may be summarized as a wavelength-dispersive optical device, such as a phasar, exhibiting flattened band pass characteristics relative to a gaussian passband having the same band width. A segmented waveguide is placed on the input wall or the output wall of the wavelength-dispersive device, and the size and placement of its segments are chosen for a flattened passband response.
In one aspect of the invention, a single-mode waveguide carrying an optical signal is coupled to the wavelength-dispersive optical device through a segmented waveguide comprising a plurality of segments of high-index materials relative to the lower-index surrounding cladding area. The segmented waveguide widens the optical field from a narrow single-mode field on the single-mode waveguide end to a broadened multi-mode field on the side of the free-space region.
The segmented waveguide may be placed on the input side of the wavelength-dispersive device or on the output side coupling at least one of the input or output ports to the device.
The invention includes a passband-flattened phasar having two optical interaction regions, for example, free-space regions, coupled by multiple waveguides of differing length.
Advantageously, the radiation is input to and output from the device on single-mode optical waveguides. The segmented waveguide may be interposed between one of the single-mode waveguides and the wavelength-dispersive device, for example, on the wall of one of the free-space regions. However, the invention is not limited to single-mode waveguides and may include a tapered waveguide, a multi-mode waveguide, or other type of optical port connected to the outer end of the segmented waveguide.
Preferably the segments are pairs of rectangular blocks symmetrically disposed about the principal axis although they may be continuous across the axis. The blocks may have constant widths along the principal axis but have differing widths transverse to the principal axis. However, other forms of the segments are possible, including more than two blocks arranged about the central axis. The central axis may bend, and the blocks may be asymmetrically disposed about an axis.
The segmented waveguide for this and other wavelength-dispersive applications may be designed with an evolutionary algorithm which evolves an initial design into an improved design by introducing fixed variations in widths of the individual segments and determining if such variations improve the passband flattening or other spectral characteristics at the other side of the wavelength-dispersive device.
The invention is particularly useful in a wavelength division multiplexing telecommunications network based on optical fiber.